Paper offset master



United States Patent 3,368,484 PAPER OFFSET MASTER William P. Fairchild,San Diego, Calif., asslgnor to Kelco Company, San Diego, Calif, acorporation of Delaware No Drawing. Filed Apr. 4, 1966, Ser. No. 539,63021 Claims. (Cl. 101-463) This invention relates to a novel direct imagepaper offset master and a method for its manufacture. More specifically,the invention pertains to a direct image paper offset master which iscoated with a compositlon contaming a relatively undegraded cellulosesulfate having a degree of substitution (D.S.) in the range of 1-3.

The requirements for a direct image paper offset master are quitestringent. The coated surface of the paper must be capable of uniformlyaccepting ink or other greasy or oily imaging materials so as to give afaithful reproduction of the image, while the non-imaged areas of thepaper offset master are kept free of ink by wetting with an etch orfountain solution. In order that the offset master function properly,its surface must be both hydrophilic and oleophilic. Also, the papercoating must be water resistant such that the coating adhesive is notdissolved by contact with water.

An object of this invention is to provide a direct image paper offsetmaster Whose surface coating contains a relatively undegraded colloidalcellulose sulfate having a degree of substitution of from 1 to 3, theterm degree of substitution having reference to the average number ofsulfate substituent groups per unit in the cellulose molecule.

A further object is to provide a method for preparing a direct imagepaper offset master whose surface coating contains a relativelyundegrated colloidal cellulose sulfate having a degree of substitutionof from 1 to 3.

Additional objects will become apparent from a reading of thespecification and claims which follow.

In forming a direct image paper offset master according to my invention,a suitable paper rawstock is first obtained. The paper sheet must be ofa high wet strength and should, therefore, be made of long fibers and behighly sized. In some instances, it may be desirable to precoat bothsides of the paper with a base coat to provide an essentially insolublesurface thereon. Highly-sized high wet strength papers are well know tothe art and do not, in themselves, form a part of my invention. See, forexample, the text entitled, Pulp and Paper, vol. II, by Casey, 2nd ed.(1960), Interscience Publishers, 250 Fifth Ave., New York, N.Y., whichdiscusses the techniques for producing highly sized, long fibercontaining paper. Moreover, the pre-coating or surface sizing of paperto provide it with an essentially insoluble surface is also a well knownprocedure. Many coatings are known which can be employed for thispurpose. To illustrate, casein in combination with suitable pigments maybe so employed as a coating. Surface coatings or surface sizings arealso discussed at length in the Casey text cited previously. The variouspigments which may be incorporated in the precoating are described inthe text entitled Paper Coating Pigments, Monograph SeriesNo. 20, by theTechnical Association of the Pulp and Paper Industry (TAPPI), 155 E.44th St., New York 17, N.Y.

The prepared sheet of paper is then coated with a suitable surfacecoating composition according to my invention. The surface coatingcomposition contains a suitable inert pigment such as various clays,calcium carbonate, titatnium dioxide, zinc oxide, and the like,dispsered in an aqueous solution of a water soluble salt of a relativelyundegraded cellulose sulfate having a D.S. of 1 to 3. This coating formsthe printing surface of the paper ofiset master. I have found thatvarying weight ratios of pigice ment to cellulose sulfate may beemployed in the paper coating. In general, I employ a weight ratio ofpigment to cellulose sulfate which may vary from about 10:1 to about3.33:1. Preferably, however, the ratio of pigment to cellulose sulfateranges from about 5:1 to about 4:1. The quantity of water employed inthe coating composition can be varied depending on the consistencydesired. In general, the water constitutes from about 60 to about andpreferably about 70 to about 75% by weight of the coating composition.The water soluble cellulose sulfate salts employed are those which inaqueous solution give a pH from in excess of 6 to about 7 or slightlyabove. The pigments employed in the surface coating may be of thevarious pigments as set forth in the TAPPI text, above. Generally, thepigment employed in the surface coating is the same as that employed inthe pre-coating.

The cellulose sulfate salts which I employ and their method ofpreparation are described in U.S. patent application, Ser. No. 467,738,filed June 28, 1965 in the name of Richard G. Schweiger and entitledProcess of Preparing a Gellable Colloidal Cellulose Sulfate and Product.U.S. application Ser. No. 467,738 is a continuationin-part of an earlierapplication Ser. No. 347,351, filed Feb. 26, 1964. The subject matter ofpatent application Ser. No. 467,738 is incorporated herein by reference.

In the cellulose sulfate salts employed, the cellulose remainsrelatively undegraded such that the sulfated ma.- terial exhibitscolloidal properties with a desired degree of substitution. The degreeof substitution (D.S.) of the cellulose sulfate is between 1.0 and 3.0.The preparation of the cellulose sulfate employed in my invention isaccomplished by first forming a complex of sulfur trioxide and a lowerN-dialkyl amide, which is used as a sulfating reagent for the cellulose.Prior to the sulfation reaction, the cellulose is preferably admixed ordiluted with at least an equal weight of the same lower N-dialkyl amideemployed in the complex. Preferably, the N-dialkyl amide is dimethylforrnamide although diethyl formamide, dimethyl acetamide, diethylacetamide, and dimethyl pro pionamide may also be used.

As described in U.S. patent application, Ser. No. 467,738, the cellulosesulfate which I employ has a D.S. of 1 to 3. The D.S. can range inexcess of 2.0 and is preferably in the range of 1.5 to 2.6. Thecellulose sulfate is soluble in water and forms gels when potassium ionsare added to aqueous solutions of the cellulose sulfate. The cellulosesulfate which I employ, when added to water, produces viscosities inexcess of 20 cps. at a concentration of 1% by weight as measured by aBrookfield Synchro Electric Viscometer, Model LVF, at 60 r.p.m. and atemperature of 25 C., and, as shown in the examples of application Ser.No. 467,73 8, the viscosities range upwardly in excess of cps. to ashigh as 500 cps. at a 1% concentration of the cellulose sulfate in theaqueous media.

The sulfating complex contains at least 1 mole of the N-dialkyl amidefor each mole of sulfur trioxide and preferably 2 moles of the N-dialkylamide are present for each mole of sulfur trioxide. In general, it isdesirable to use a weight of SO -N-dialkyl amide complex which is atleast two times the weight of the cellulose.

The cellulose sulfate salts employed in my invention are formed byneutralizing the free acid through reaction with a suitable base. Thecellulose sulfate is a halfester of sulfuric acid. Thus, one of thehydrogen ions originally present in the sulfuric acid is still free toreact to give salts. As defined, the salts employed are water solubleand in aqueous solution give a pH of in ex cess of 6 to essentiallyneutral. Suitable salts, for example, are obtained by reaction of theacid with various bases to give alkali metal salts, such as sodium,lithium, potassium, ammonium, magnesium and the like, or with organiccations, especially short chain alkyl and alkylol substituted ammonium,such as methylammonium, triethanolamrnonium, dimethyLbenZylammOnium, andthe like. Preferably, the salt is an alkali metal salt such as sodium orpotassium.

The coating containing a suitable inert pigment dispersed in an aqueoussolution of a salt of cellulose sulfate, as defined above, may beapplied to the paper by means of any of the various conventional coatingtechniques such as the use of a roll, flexiblade, or air knife coater.In order to prevent curl, it may be desirable in some cases to apply thecoating to the back side of the sheet also.

Following coating, the coated sheet is dried and is then treated with anaqueous solution of a soluble potassium salt. Various potassium saltsmay be employed such as potassium chloride, potassium sulfate, potassiumcarbonate, potassium bicarbonate, potassium citrate, potassium bromide,potassium pyrophosphate, potassium tartrate, potassium acetate, and thelike. Preferably, potassium chloride is employed as the potassium salt.The concentration of the potassium ion in the aqueous treating solutionis not critical so long as a sufficient amount of potassium ion ispresent to insolubilize the cellulose sulfate such that it is waterresistant. When employing an aqueous solution of potassium chloride, Igenerally use approximately a solution. Following treatment of thecoated sheet with an aqueous solution containing a potassium salt, thetreated sheet may then be water washed, if desired, and supercalenderedor subjected to equivalent treatment to give the sheet the degree ofgloss desired.

To further illustrate my invention, there are presented the followingexamples in which all parts and percentages are by weight unlessotherwise illustrated.

Example I In 41.3 grams of water were dispersed 96.7 grams of No. 2kaolin coating clay. To this dispersion were added 242.7 grams of waterand 19.3 grams of a relatively undegraded sodium cellulose sulfatehaving colloidal properties and a D8. of about 2.2. The mixture was thenagitated for a sufficient length of time to allow the sodium cellulosesulfate to dissolve. The total solids contained in the coating were 29%and the weight ratio of the sodium cellulose sulfate to the kaolin claywas about :100.

The coating was applied to 18 lb./ream litho rawstock using a No. 20wire wrapped coating rod. The coated sheet was allowed to air dry afterwhich it was passed through a 3% potassium chloride bath and then rinsedwith distilled water. Drying was then effected by passing the sheetthrough a drum dryer at 180 F. with a 3 minute hold time. The sheetexhibited excellent Web rub properties while at the same time it did notrepel water. The coated surface accepted images in a satisfactory mannerand was judged to be suitable for offset printing.

Example II To 240 lbs. of water were added 10 lbs. of relativelyundegraded sodium cellulose sulfate having colloidal properties and aD5. of about 2.2. After the sodium cellulose sulfate had dissolved inthe water, 50 lbs. of No. 2 kaolin coating clay were added thereto toform a dispersion. The resulting mixture was agitated until a constantminimum viscosity was obtained, which was 2700 cps. at 99 F. Theviscosity of the mixture was determined with a Brookfield Model LVFViscometer using a No. 4 spindle rotating at 60 rpm. The total solidscontained in the coating mixture were 20% and the weight ratio of sodiumcellulose sulfate to clay was 20:100.

After the coating mixture had attained a constant minimum viscosity, itwas applied to a 20 lb./ream map rawstock by means of an air-knifecoater. The coating speed was 500 feet per minute. Two coating passeswere made on the top (printing) side and one coating pass was made 4 onthe bottom side. The total coating pick-up was 6.5 lbs/ream with 6.0lbs. on the top side and 0.5 lb. on the bottom side.

The printing surface was then treated with a 3% aqueous solution ofpotassium chloride using the air-knife coater. The coater speed was 200feet per minute. Following treatment of the paper with potassiumchloride, it was then dried and supercalendered in the normal manner togive the paper the desired gloss. The finished paper had the desiredproperties for use as an offset master paper.

In the foregoing examples, the paper employed was a high Wet strengthpaper which had been previously sized in the normal manner. Although thesodium salt of an essentially undegraded cellulose sulfate was employedin these examples, satisfactory paper offset masters are obtained byemploying any of the various water soluble salts of a relativelyundegraded cellulose sulfate having colloidal properties and a D8. ofl-3 as enumerated previously. Moreover, potassium salts, as definedpreviously, other than potassium chloride, may be employed in obtaininga satisfactory paper offset master.

It should be emphasized that the cellulose sulfate or salt thereof whichI employ is quite unlike previously available cellulose sulfate. Thematerial which I employ is relatively undegraded such that its salts,have colloidal properties. Previously obtainable cellulose sulfate had adegraded molecular structure due to the severity of the sulfationprocedure. Such material does not have colloidal properties and is notsuitable for preparing a paper offset master according to my invention.

In use, a greasy image is made on the paper offset master of myinvention by any means such as ty'ping, drawing or writing. The image ismade with a suitable ink or pencil or equivalent means. The surface areawhich contains the image is grease receptive and will accept ink.

Following the placing of an image on the surface of the offset master,the surface is treated with an etching or fountain solution. Thenon-imaged areas of the offset master are both hydrophilic and waterresistant. Thus, the non-imaged areas are wetted with the solution suchthat they will repel ink. Conversely, the image areas of the offsetmaster are hydrophobic and are not wetted by the etching or fountainsolution. These areas are grease receptive and will retain ink.

Many suitable etching solutions are available. As an example, a gumarabic solution with barium nitrate and water is available. The presentinvention is not limited to use of any particular fountain solution,many of which are known to the prior art.

After treatment of the offset master with fountain solution, the masteris placed on an offset printing press. A thin film of an oil base ink isthen applied to the master as, for example, by a roller. The ink isaccepted by the imaged areas of the master which are oleophilic and isresisted by the non-imaged areas which are wetted with the etch orfountain solution. The ink image is used in offset printing in a wellknown manner by transfer to a drum and then to the paper.

Various image techniques can be employed in the use of the paper offsetmaster of my invention. For example, the surface coating for the offsetmaster can also contain ammonium bichromate to make it light sensitive.The coated master can then be 'placed in a printing frame in contactwith a negative. On exposure to a strong light source for a suitableperiod of time, the plate is removed from the frame or other holdingmeans and is separated from the negative. Following this, the developingink is applied to the plate and the plate is developed out in a knownmanner. Any excess developing ink is removed and the plate is thenpreferably put under running water to remove the ink on all the areasexcept those areas which have been hardened and made isoluble by thelight. The hardened and insolubilized areas are receptive to printinginks. Following this treatment, the plate is then treated with an etchor fountain solution in the manner described previously, then a thinfilm of ink is laid down on the plate, and it is used in the knownmanner in an offset printing press.

Also, the paper offset master of my invention may be madephotosensitive. To do this, silver salts, such as silver bromide orsilver chloride or silver iodide or any combination thereof, are addedto the surface coating along with the cellulose sulfate salt, water, andpigment in the amount described previously. Following drying of thecoating, etc., the paper offset master is ready for exposure, developingand fixing operations and for further use. In use, the plate, togetherwith the negative is subjected to alight of suitable intensity. Aftersensitization, the plate may be either used for contact printing orenlargement and after a suitable exposure the usual photographic stepsof developing and fixing are carried out and the exposed silver saltsare reduced to metallic silver.

The reduced metallic silver now present in the developed and fixed plateis not grease receptive and will not accept greasy inks. Thus, thereduced metallic silver may then be converted into a chromate salt bythe use of a suitable bleaching agent. After the plate has been treatedwith the bleach, the image is grease receptive. After the bleachingoperation, the plate is treated with the developing ink and oncompletion of that treatment is ready for use as an offset printingplate.

As shown by the foregoing description, my invention is a broad onehaving a wide variety of applications. Although I have described myinvention by reference to specific examples, I do not intend myinvention to be limited except by the lawful scope of the appendedclaims.

I claim:

1. A process for forming a direct image paper offset master, saidprocess comprising coating a high wet strength paper with an aqueouscoating composition containing an inert pigment and a water soluble saltof a colloidal cellulose sulfate having a D.S. of 1 to 3 and having aviscosity at a 1% concentration in an aqueous media in excess of 2:0cps. as measured by a Brookfield Synchro Electric Viscometer, Model LVF,at 6 0 rpm. and a temperature of 25 C., said salt being characterized asone which in aqueous solution gives a pH from in excess of 6 to about 7,drying said coated sheet and treating the sheet with an aqueous solutionof a soluble potassium salt in a sutficient amount to insolubilize thesaid cellulose sulfate salt such that it is water resistant.

2. The process of claim 1 wherein said cellulose sulfate salt is presentin said coating composition at a weight ratio from about 10:1 to about3.33:1 with respect to said pigment.

3. The method of claim 2 wherein said cellulose sulfate salt is presentin said coating composition at a weight ratio from about 5:1 to about4:1 with respect to said pigment.

4. The process of claim 2 wherein said coating composition contains fromabout 60 to about 85% by weight of water.

5. The process of claim 3 wherein said coating composition contains fromabout 70 to about 75% by weight of water.

6. The process of claim 3 wherein said direct image paper offset masteris supcrcalendered to give it the desired degree of gloss followingtreatment with said soluble potassium salt.

7. The process of claim 1 wherein said water soluble cellulose sulfatesalt is an alkali metal salt.

3. The process of claim 7 wherein said water soluble cellulose sulfatesalt is a sodium salt.

9. The process of claim 7 wherein said water soluble cellulose sulfatesalt is a potassium salt.

10. The process of claim 1 wherein said soluble potassium salt ispotassium chloride.

11. The process of claim 10' wherein said potassium chloride salt ispresent in said aqueous solution at a concentration of about 10% byweight.

12. The process of claim 1 wherein said direct image paper offset masteris supercalendered to give it the desired degree of gloss followingtreatment with said soluble potassium salt.

13. The process of claim 1 wherein said colloidal cellulose sulfate hasa D.S. in excess of 2.

14. A direct image paper offset master prepared according to the methodof claim 6.

15. A direct image paper offset master prepared according to the methodof claim 12.

16. A coating composition-comprising an inert pigment, a water solublesalt of a colloidal cellulose sulfate having a degree of substitution inthe range of 1 to 3 which forms gels in aqueous solution on the additionthereto of potassium ions and said salt having a viscosity at a 1%concentration in an aqueous media in excess of 20 cps. as measured by aBrookfield Synchro Electric Viscometer, Model LVF, at 60 r.-p.m. and atemperature of 25 C., said cellulose sulfate salt being characterized asproducing a pH in aqueous solution ranging from in excess of 6 to about7, with the weight ratio of said cellulose sulfate salt to said pigmentranging from about 10:1 to about 3.33:1, and from about 60 to about 85%by weight of water.

17. The coating composition of claim 16 wherein the weight ratio of saidcellulose sulfate salt to said inert pigment ranges from about 5:1 toabout 4:1, and the water content ranges from about to about by weight.

18. The composition of claim 16 wherein said water soluble cellulosesulfate salt is an alkali metal salt.

19. The composition of claim 18 wherein said cellulose sulfate salt is asodium salt.

20. The composition of claim 18 wherein said cellulose sulfate salt is apotassium salt.

21. The coating composition of claim 16 wherein said colloidal cellulosesulfate has a D.S. in excess of 2.

References Cited UNITED STATES PATENTS 2,502,783 5/1950 Erickson 1171572,776,912 1/ 1957 Gregory. 2,953,088 9/1960 Newman 101--149.2

FOREIGN PATENTS 724,907 2/ 1955 Great Britain.

DAVID KLEIN, Primary Examiner.

ROBERT E. PULFREY, Examiner.

I. A. BELL, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,368,484 February 13, 1968 William P. Fairchild It is certified thaterror appears in the above identified patent and that said LettersPatent are hereby corrected as show below:

Column 5, lines 49 and 50, S3 and 54, and column 6,

lines 34 and 35, "cellulose, sulfate salt", each occurrence, should readpigment "same column 5, lines 52 and 5.6, and same column 6, line 35,"pigment each occurrence, should read cellulose sulfate salt same column6, line 39 "cellulose sulfate salt" should read inert pigment "3 lines.39 and 40 "inert pigment" should read cellulose sulfate salt Signed andsealed this 2nd day of September 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. JR.

Attesting Officer Commissioner of Patents

1. A PROCESS FOR FORMING A DIRECT IMAGE PAPER OFFSET MASTER, SAIDPROCESS COMPRISING COATING A HIGH WET STRENGTH PAPER WITH AN AQUEOUSCOATING COMPOSITIO CONTAINING AN INERT PIGMENT AND A WATER SOLUBLE SALTOF A COLLOIDAL CELLULOSE SULFATE HAVING A D.S. OF 1 TO 3 AND HAVING AVISCOSITY AT A 1% CONCENTRATION IN AN AQUEOUS MEDIA IN EXCESS OF 20 CPS.AS MEASURED BY A BROOKFIELD SYNCHRO ELECTRIC VISCOMETER, MODEL LVF, AT60 R.P.M. AND A TEMPERATURE OF 25*C., SAID SALT BEING CHARACTERIZED ASONE WHICH IN AQUEOUS SOLUTION GIVES A PH FROM IN EXCESS OF 6 TO ABOUT 7,DRYING SAID COATED SHEET AND TREATING THE SHEET WITH AN AQUEOUS SOLUTIONOF A SOLUBLE POTASSIUM SALT IN A SUFFICIENT AMOUNT TO INSOLUBILIZE THESAID CELLULOSE SULFATE SALT SUCH THAT IT IS WATER RESISTANT.